1,782 research outputs found
Three-integral oblate galaxy models
A simple numerical scheme is presented for the construction of three-integral
phase-space distribution functions for oblate galaxy models with a
gravitational potential of St\"{a}ckel form, and an arbitrary axisymmetric
luminous density distribution. The intrinsic velocity moments can be obtained
simultaneously with little extra effort. The distribution of the inner and
outer turning points of the short-axis tube orbits that are populated can be
specified freely, and is chosen in advance. The entire distribution function is
then derived from the density by an iterative scheme that starts from the
explicitly known distribution function of the thin-orbit (maximum streaming)
model, in which only the tubes with equal inner and outer turning points are
populated. The versatility and limitations of this scheme are illustrated by
the construction of a number of self-consistent three-integral flattened
isochrone models of Kuzmin--Kutuzov type, and by investigation of special cases
where the scheme is tractable analytically. This includes the behaviour of the
distribution functions in the outer regions of the models. The scheme converges
rapidly for models containing orbits with ratios of the outer to inner turning
point as large as ten, and is particularly suited for the construction of
tangentially anisotropic flattened models, self-consistent as well as
non-consistent. The algorithm simplifies in the disk and spherical limit, and
can be generalized to triaxial models.Comment: uuencoded gziped PostScript, 21 pages without figures. PostScript
with figures available from http://www.strw.leidenuniv.nl/dynamics/Papers/ or
ftp://ftp.strw.leidenuniv.nl/pub/dynamics/Papers/RobijnDeZeeuw95.tar.Z
Accepted by MNRA
Dark matter in elliptical galaxies
We present measurements of the shape of the stellar line-of-sight velocity
distribution out to two effective radii along the major axes of the four
elliptical galaxies NGC 2434, 2663, 3706, and 5018. The velocity dispersion
profiles are flat or decline gently with radius. We compare the data to the
predictions of f=f(E,L_z) axisymmetric models with and without dark matter.
Strong tangential anisotropy is ruled out at large radii. We conclude from our
measurements that massive dark halos must be present in three of the four
galaxies, while for the fourth galaxy (NGC 2663) the case is inconclusive.Comment: 15 pages, uuencoded compressed PostScript, includes 3 figure
Resonant Orbits in Triaxial Galaxies
Box orbits in triaxial potentials are generically thin, that is, they lie
close in phase space to a resonant orbit satisfying a relation of the form
l\omega_1 +m\omega_2+n\omega_3=0 between the three fundamental frequencies.
Resonant orbits are confined to a membrane; they play roughly the same role, in
three dimensions, that closed orbits play in two. Stable resonant orbits avoid
the center of the potential; orbits that are thick enough to pass near the
center are typically stochastic. Very near the center, where the gravitational
potential is dominated by the black hole, resonant orbits continue to exist,
including at least one family whose elongation is parallel to the long axes of
the triaxial figure.Comment: 20 Latex pages, 11 Postscript figures. Submitted to The Astronomical
Journa
Mapping young stellar populations towards Orion with Gaia DR1
We use the first data release of the Gaia mission to explore the three
dimensional arrangement and the age ordering of the many stellar groups towards
the Orion OB association, aiming at a new classification and characterization
of the stellar population. We make use of the parallaxes and proper motions
provided in the Tycho Gaia Astrometric Solution (TGAS) sub-set of the Gaia
catalogue, and of the combination of Gaia and 2MASS photometry. In TGAS we find
evidence for the presence of a young population, at a parallax , loosely distributed around some known clusters: 25 Ori,
Ori and Ori, and NGC 1980 ( Ori). The low mass
counterpart of this population is visible in the color-magnitude diagrams
constructed by combining Gaia and 2MASS photometry. We study the density
distribution of the young sources in the sky. We find the same groups as in
TGAS, and also some other density enhancements that might be related to the
recently discovered Orion X group, the Orion dust ring, and to the
Ori complex. We estimate the ages of this population and we infer the presence
of an age gradient going from 25 Ori (13-15 Myr) to the ONC (1-2 Myr). We
confirm this age ordering by repeating the Bayesian fit using the Pan-STARRS1
data. The estimated ages towards the NGC 1980 cluster span a broad range of
values. This can either be due to the presence of two populations coming from
two different episodes of star formation or to a large spread along the line of
sight of the same population. Our results form the first step towards using the
Gaia data to unravel the complex star formation history of the Orion region in
terms of the different star formation episodes, their duration, and their
effects on the surrounding interstellar medium.Comment: 17 pages, 17 figure
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